The present invention is related to the field of flyback power supplies. More particularly, the present invention is related to stable flyback power supplies which include an output sensing feedback control loop and wherein the power supply remains stable even when operated in a continuous excitation mode with respect to current excitation of the primary winding of a flyback transformer in the power supply.
In known flyback power supplies, pulsed current excitation is supplied to the primary winding of a flyback transformer and a secondary winding of the transformer is coupled, typically through a rectifier, to a load. The rectified output is sensed and provided by a feedback path as an input to a drive circuit which provides drive signals that determine the primary winding current excitation pulses. The drive signals vary in accordance with the sensed output so as to maintain the output at a desired level, thus regulating the output.
Typically these flyback power supplies are operated in a discontinuous mode for stability reasons. In a discontinuous mode, magnetic flux will increase in the transformer during primary winding current pulses, and between primary winding current pulses the flux decreases to a substantially zero value. Typically the decrease of flux to zero is abrupt, the flyback transformers operated in the discontinuous mode typically generate substantial radio frequency interference (RFI). In addition, typically large primary winding current pulses must be utilized in order to store a substantial amount of energy for subsequent transfer to the secondary winding, or else the frequency of the primary excitation pulses must be increased.
If the flyback power supply is operated in a continuous mode such that the transformer flux has a substantial non-zero magnitude prior to the drive circuit causing an increase in flux to store additional energy, less primary current can be utilized and less RF interference is generated. Also a lower excitation frequency can be used if desired. However, operating in a continuous mode typically results in stability problems for the power supply, thus preventing the utilization of a continuous mode flyback power supply or requiring tight controls on the system gain, the input signal magnitude and the frequency of primary current excitation in order to maintain limited stability. However, due to load variation, typically this stability cannot be maintained. Therefore, substantially all flyback power supplies are operated in the discontinuous mode.